1680s (in hump-backed), of uncertain origin; perhaps from Dutch homp "lump," from Middle Low German hump "bump," from Proto-Germanic *hump-, from PIE *kemb- "to bend, turn, change, exchange" (see change (v.)). Replaced, or perhaps influenced by, crump, from Old English crump.
A meaning attested from 1901 is "mound in a railway yard over which cars must be pushed," which might be behind the figurative sense of "critical point of an undertaking" (1914). By 1957, hump day was in use in reference to the mid-point of a training program or course; it was extended to "Wednesday," as the mid-point of the work-week, by 1987.
c. 1400, "bulging, convex," from Late Latin gibbus "hunchbacked," from Latin gibbus "a hump, a hunch," as an adjective, "bulging," from Proto-Italic *gifri- "hump," *gifro- "hump-backed," of uncertain origin. De Vaan suggests a PIE *geibh-, with possible cognates in Lithuanian geibus "gawky, plump," geibstu, geibti "become weak;" Norwegian dialect keiv "slanted, wrong," keiva "left hand," perhaps united by a general sense of "bodily defect." Of the moon from early 15c.; also used from 15c. of hunchbacks. Related: Gibbosity.
French nursery rhyme hero (the rhyme first attested in English 1810), earlier "a short, clumsy person of either sex" (1785), probably a reduplication of Humpty, a pet form of Humphrey. Compare Georgy-porgy, etc. Originally, humpty-dumpty was a drink (1690s), "ale boiled with brandy," probably from hump and dump, but the connection is obscure and there might not be one.
'It's very provoking,' Humpty Dumpty said, ... 'to be called an egg — very!' ["Through the Looking-Glass," 1872]
