Etymology
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libido (n.)
"psychic drive or energy, usually associated with sexual instinct," 1892, carried over untranslated in English edition of Krafft-Ebing's "Psychopathia Sexualis"; and used in 1909 in A.A. Brill's translation of Freud's "Selected Papers on Hysteria" (Freud's use of the term led to its popularity); from Latin libido, lubido "desire, eagerness, longing; inordinate desire, sensual passion, lust," from libere "to be pleasing, to please," from PIE root *leubh- "to care, desire, love" (source also of love).
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libidinal (adj.)
in psychology jargon, 1922, in Joan Riviere's translation of Freud, from combining form of libido (Latin genitive libidinis) + -al (1).
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ad lib 

also ad lib., 1811 as a musical instruction, shortened from Latin ad libitum "to (one's) pleasure, as much as one likes" (c. 1600), from ad "to" (see ad-) + libitum "pleasure," accusative of libere "to please" (see libido). As a noun from 1825; as a verb by 1915.

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libidinous (adj.)
"lustful," mid-15c., from Old French libidineus "sinful, lusty" (13c., Modern French libidineux) or directly from Latin libidinosus "full of desire, lustful," from libido "pleasure, desire, sensual passion, lust" (see libido). Related: Libidinously; libidinousness; libidinosity. These are older in English than libido, libidinal, which are from modern psychology.
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*leubh- 

Proto-Indo-European root meaning "to care, desire, love."

It forms all or part of: belief; believe; furlough; leave (n.) "permission, liberty granted to do something;" leman; libido; lief; livelong; love; lovely; quodlibet.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit lubhyati "desires," lobhaya- "to make crazy;" Persian ahiftan "to be tangled, be hit down, be in love;" Latin lubet, later libet "pleases," libido, lubido "desire, longing; sensual passion, lust;" Old Church Slavonic l'ubu "dear, beloved," ljubiti, Russian ljubit' "to love;" Lithuanian liaupsė "song of praise;" Old English lufu "feeling of love; romantic sexual attraction," German Liebe "love," Gothic liufs "dear, beloved."

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cathexis (n.)

"concentration or accumulation of mental energy," 1922, from Latinized form of Greek kathexis "holding, retention," from PIE root *segh- "to hold." Used by psychologists to render Freud's (Libido)besetzung.

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