mid-15c., "concealed, secret," from Latin latentem (nominative latens) "lying hid, concealed, secret, unknown," present participle of latere "lie hidden, lurk, be concealed," from PIE *late-, suffixed form of root *lādh- "to be hidden" (source also of Greek lēthē "forgetfulness, oblivion," lēthargos "forgetful," lathre "secretly, by stealth," lathrios "stealthy," lanthanein "to be hidden;" Old Church Slavonic lajati "to lie in wait for"). Meaning "dormant, undeveloped" is from 1680s, originally in medicine.
mythical river of Hades (whose water when drunk caused forgetfulness of the past), in Homer, a place of oblivion in the lower world; from Greek lēthē, literally "forgetfulness, oblivion," from PIE root *ladh- "be hidden" (see latent). Related to lēthargos "forgetful" and cognate with Latin latere "to be hidden." Also the name of a personification of oblivion, a daughter of Eris. Related: Lethean.
"In early Greek mythology, the spirit of revenge, that prompts the members of a family to commit fresh crimes to obtain satisfaction" [Encyclopaedia Britannica, 1941]. The name also was used of the evil genius which drives a man to sin and of a man so driven. A Greek word of uncertain origin. The traditional guess is that it is literally "the unforgetting," from a- "not" (see a- (3)) + root of lathein "to forget," aorist of lanthanein "to lie hidden, escape notice," from PIE root *ladh- "to be hidden" (see latent). Or else it might be connected with alaomai "to wander, roam," figuratively "to be distraught." As a proper name, in Greek tradition a son of Neleus and Chloris; brother of Nestor, he was slain by Herakles.
late 14c., litarge, "state of prolonged torpor or inactivity, inertness of body or mind," from Medieval Latin litargia, from Late Latin lethargia, from Greek lēthargia "forgetfulness," from lēthargos "forgetful," apparently etymologically "inactive through forgetfulness," from lēthē "a forgetting, forgetfulness" (see latent) + argos "idle" (see argon). The form with -th- is from 1590s in English. The Medieval Latin word also is the source of Old French litargie (Modern French léthargie), Spanish and Italian letargia.
1833, "one who or that which develops," agent noun from develop. Photography use in reference to the chemical bath used to bring out the latent image is attested from 1869; meaning "speculative builder" is by 1938.