mid-14c., relien, "to gather, assemble" an army, followers, a host, etc. (transitive and intransitive), from Old French relier "assemble, put together; fasten, fasten again, attach, rally, oblige," from Latin religare "fasten, bind fast," from re-, here perhaps an intensive prefix (see re-), + ligare "to bind" (from PIE root *leig- "to tie, bind").
The older sense now are obsolete. The meaning "depend on with full trust and confidence, attach one's faith to" a person or thing is from 1570s, perhaps via the notion of "rally to, fall back on." Typically used with on, perhaps by influence of unrelated lie (v.2) "rest horizontally." Related: Relied; relying.
The verb rely, in the orig. sense 'fasten, fix, attach,' came to be used with a special reference to attaching one's faith or oneself to a person or thing (cf. 'to pin one's faith to a thing,' 'a man to tie to,' colloquial phrases containing the same figure); in this use it became, by omission of the object, in transitive, and, losing thus its etymological associations (the other use, 'bring together again, rally,' having also become obsolete), was sometimes regarded, and has been by some etymologists actually explained, as a barbarous compound of re- + E. lie (1) rest, .... But the pret. would then have been *relay, pp. *relain. [Century Dictionary]
c. 1200, cwitance, quitaunce, "payment, compensation;" c. 1300, "a discharge from a debt or an obligation," from Old French quitance (Modern French quittance), from quiter "clear, establish one's innocence;" also transitive, "release, let go, relinquish, abandon" (12c.), from quite "free, clear, entire, at liberty; discharged; unmarried," from Medieval Latin quitus, quittus, from Latin quietus "free" (in Medieval Latin "free from war, debts, etc."), also "calm, resting" (from PIE root *kweie- "to rest, be quiet"). The Middle English word also is in part from Medieval Latin quittantia, a variant of quietantia.
On, Upon. These words are in many uses identical in force, but upon is by origin (up + on) and in use more distinctly expressive of motion to the object from above or from the side. On has the same force, but is so widely used in other ways, and so often expresses mere rest, that it is felt by careful writers to be inadequate to the uses for which upon is preferred. [Century Dictionary, 1895]
"stiff grass," Old English beonet (attested only in place names), from West Germanic *binut- "rush, marsh grass" (source also of Old Saxon binet, Old High German binuz, German Binse "rush, reed"), which is of unknown origin. An obsolete word, but surviving in place names (such as Bentley, from Old English Beonet-leah; and Bentham).
The verdure of the plain lies buried deep
Beneath the dazzling deluge; and the bents,
And coarser grass, upspearing o'er the rest,
Of late unsightly and unseen, now shine
Conspicuous, and, in bright apparel clad
And fledg'd with icy feathers, nod superb.
[Cowper, "The Winter-Morning Walk," from "The Task"]
BOATSWAIN. The warrant officer who in the old Navy was responsible for all the gear that set the ship in motion and all the tackle that kept her at rest. [Sir Geoffrey Callender, "Sea Passages," 1943]
He also summons the hands to their duties with a silver whistle. Phonetic spelling bo'sun/bosun is attested from 1840. Fowler  writes, "The nautical pronunciation (bō'sn) has become so general that to avoid it is more affected than to use it."
mid-15c., "to be attached to as a condition or cause, be a conditional effect or result," a figurative use, also literal, "to hang, be sustained by being attached to something above;" from Old French dependre, literally "to hang from, hang down," and directly from Latin dependere "to hang from, hang down; be dependent on, be derived," from de "from, down" (see de-) + pendere "to hang, cause to hang; weigh" (from PIE root *(s)pen- "to draw, stretch, spin").
From c. 1500 as "to rely, rest in full confidence or belief;" from 1540s as "be sustained by, be dependent (on)." Related: Depended; depending.
Modern French for "yes," from Old French oïl "yes," at first two words meaning "yes, he," or "yes, they," which gradually came to mean simply "yes." From the Latin phrase hoc ille "yes," literally "this he, so he" (did or said).
The French originally said "yes, I," "yes, you," "yes, we," etc., where the pronoun was the subject of an unexpressed verb easily supplied from the question. [C.H.C. Wright, "A History of French Literature," Haskell House, 1969]
Thus the o is from Latin hoc "this," and the rest of the word is from the Latin personal pronoun ille "he" (in Vulgar Latin illi which is also "they"). Old French also had o alone as "yes." Compare Languedoc.
1590s, "recurring in sevens or on every seventh;" 1640s, "of or suitable for the Sabbath," from Latin sabbaticus, from Greek sabbatikos "of the Sabbath," from sabbaton (see Sabbath). By 1836 as "characterized by rest or cessation from labor or tillage." Other adjectives from Sabbath include Sabbatary, Sabbatine.
The noun meaning "a year's absence granted to researchers" (originally one year in seven, to university professors) is from 1934, short for sabbatical year, etc., which was recorded by 1886 (the thing itself is attested from 1880, at Harvard), a term perhaps suggested by the sabbatical year (1590s) in Mosaic law, the seventh year, in which land was to remain untilled and non-foreign debtors and slaves released.