formerly also endulge, 1630s, "to grant as a favor;" 1650s, "to treat with unearned favor" (in reference both to persons and desires), a back-formation from indulgence (q.v.), or directly from Latin indulgere "be complaisant, be indulgent, yield; give oneself up to." Related: Indulged; indulging; indulgingly.

mid-14c., in the Church sense, "a freeing from temporal punishment for sin, remission from punishment for sin that remains due after absolution," from Old French indulgence or directly from Latin indulgentia "complaisance, a yielding; fondness, tenderness, affection; remission," from indulgentem (nominative indulgens) "indulgent, kind, tender, fond," present participle of indulgere "be kind; yield, concede, be complaisant; give oneself up to, be addicted," a word of uncertain origin. It is evidently a compound, and the second element appears to be from PIE root *dlegh- "to engage oneself, be or become fixed." The first element could be in- "in" for a sense of "let someone be engaged" in something, or in- "not" for a total sense of "not be hard toward" someone.

Sense of "leniency, forbearance of restraint or control of another, gratification of desire or humor" is attested from late 14c. That of "yielding to one's inclinations" (technically self-indulgence) in English is from 1630s. In British history, Indulgence also refers to grants of certain liberties to Nonconformists under Charles II and James II, as special favors rather than legal rights. The sale of indulgences in the original Church sense was done at times merely to raise money and was widely considered corrupt; the one in 1517 helped to spark the Protestant revolt in Germany.

Middle English pleien, from Old English plegan, plegian "move lightly and quickly, occupy or busy oneself, amuse oneself; engage in active exercise; frolic; engage in children's play; make sport of, mock; perform music," from Proto-West Germanic *plegōjanan "occupy oneself about" (source also of Old Saxon plegan "vouch for, take charge of," Old Frisian plega "tend to," Middle Dutch pleyen "to rejoice, be glad," German pflegen "take care of, cultivate"), which is apparently connected to the root of plight (v.), but the ultimate etymology is uncertain and the phonetic development is difficult to explain.

Meaning "to take part in" a martial or athletic game is from c. 1200. It has been opposed to work (v.) since late 14c. Meaning "perform or act on the stage" (transitive) is by late 14c., as are the senses of "take the role of" and "make a pretense of, make believe" and "act thoughtlessly or wantonly." Sense of "put forward, move, throw, lay on the table, etc." in the course of a game or contest is by 1560s of chess pieces, 1670s of playing cards. Sense of "operate or cause to operate with continuous or repeated action" is from 1590s. Meaning "to cause (a recording) to reproduce what is on it" is by 1903, probably from the "make music" sense. Related: Played; playing.

Many expressions are from the stage, sports and games, or music, and it is not always easy to say which is from which. To play up "emphasize" is from 1909 (perhaps originally "play music more vigorously"); to play down "minimize" is from 1930; to play along "pretend to agree or cooperate" is from 1929. To play fair "be nice" is from mid-15c. To play house as a children's activity is from 1958.

To play for keeps is from 1861, originally of marbles or other children's games with tokens. To play (something) safe is from 1911; to play favorites is attested from 1902.  To play second fiddle in the figurative sense is from 1809 ("Gil Blas"). To play into the hands (of someone) "act in such a way as to give the advantage to one's opponent or a third party" is from 1705. For play the _______ card see card (n.1). For play the field see field (n.). To play with oneself "masturbate" is from 1896 (to play with "have sexual intercourse with" is from mid-13c.). Playing-card "one of a pack of cards used for playing games" is from 1540s.

mid-14c., plegge, "surety, bail," from Old French plege (Modern French pleige) "hostage, security, bail," also Anglo-Latin plegium, both probably from Frankish *plegan "to guarantee," from *pleg-, a West Germanic root meaning "have responsibility for" (source also of Old Saxon plegan "vouch for," Middle Dutch plien "to answer for, guarantee," Old High German pflegan "to care for, be accustomed to," Old English pleon "to risk the loss of, expose to danger"), from PIE root *dlegh- "to engage oneself, be or become fixed" [Watkins].

From late 14c. as "person who goes surety or gives bail for another;" late 15c. (Caxton) as "personal property given as surety for a debt or engagement. By 1520s as "a token or sign of favor, agreement, etc.

Meaning "allegiance vow attested by drinking with another" is from 1630s. Sense of "solemn promise, one's word given or considered as security for the performance (or refraining from) an act" is recorded by 1814, though this notion is from 16c. in the verb. Weekley notes the "curious contradiction" in pledge (v.) "to toast with a drink" (1540s) and pledge (n.) "the vow to abstain from drinking" (1833). Meaning "student who has agreed to join a fraternity or sorority" dates from 1901.

"to pledge, engage by solemn promise" (obsolete except in archaic plight one's troth), Middle English plighten, from Old English pligtan, plihtan "to endanger, imperil, compromise," verb form of pliht (n.) "danger, risk" (see plight (n.2)), from Proto-Germanic *plehti-, which ultimately is perhaps from PIE root *dlegh- "to engage oneself, be or become fixed," or else a substratum word. The notion is "to put (something -- honor, troth) in danger or risk of forfeiture;" it is rarely used of physical things. Related: Plighted; plighting.

in law, "recovery of goods (by someone) taken from him, upon posting of security; temporary restoration of confiscated property pending a legal hearing," mid-15c., from Anglo-French replevin (mid-14c.) and Anglo-Latin (13c.) replevina, from Old French replevir (v.) "to pledge, protect, warrant," from re- "back, again" (see re-) + plevir, a word of uncertain origin; perhaps related to pledge (v.). The corresponding verb is replevy (1550s). Related: Repleviable.

mid-13c., "solemn pledge, promise," usually concerning a serious matter and involving risk or loss in default, from Old English pliht "danger, risk, peril, damage," from Proto-Germanic *pleg- (source also of Old Frisian plicht "danger, concern, care," Middle Dutch, Dutch plicht "obligation, duty," Old High German pfliht, German Pflicht "obligation, duty," which is perhaps from PIE root *dlegh- "to engage oneself, be or become fixed," or it might be a substratum word. Compare Old English plihtere "look-out man at the prow of a ship," plihtlic "perilous, dangerous."