fleet (v.)youdaoicibaDictYouDict[fleet 词源字典]
Old English fleotan "to float; drift; flow, run (as water); swim; sail (of a ship)," from Proto-Germanic *fleutan (cognates: Old Frisian fliata, Old Saxon fliotan "to flow," Old High German fliozzan "to float, flow," German fliessen "to flow, run, trickle" (as water), Old Norse fliota "to float, flow"), from PIE root *pleu- "to flow, swim" (see pluvial).

Meaning "to glide away like a stream, vanish imperceptibly" is from c. 1200; hence "to fade, to vanish" (1570s). Related: Fleeted; fleeting.[fleet etymology, fleet origin, 英语词源]
imperceptible (adj.)youdaoicibaDictYouDict
early 15c., from French imperceptible (early 15c.), from Medieval Latin imperceptibilis, from assimilated form of in- "not, opposite of" (see in- (1)) + perceptibilis (see perceptible). Related: Imperceptibly. OED marks imperceivable as "Now rare."
meld (v.)youdaoicibaDictYouDict
"to blend together, merge, unite" (intransitive), by 1910, of uncertain origin. OED suggests "perh. a blend of MELT v.1 and WELD v." Said elsewhere to be a verb use of melled "mingled, blended," past participle of dialectal mell "to mingle, mix, combine, blend."
[T]he biplane grew smaller and smaller, the stacatto clatter of the motor became once more a drone which imperceptibly became melded with the waning murmur of country sounds .... ["Aircraft" magazine, October 1910]
But it is perhaps an image from card-playing, where the verb meld is attested by 1907 in a sense of "combine two cards for a score:"
Upon winning a trick, and before drawing from the stock, the player can "meld" certain combinations of cards. [rules for two-hand pinochle in "Hoyle's Games," 1907]
The rise of the general sense of the word in English coincides with the craze for canasta, in which melding figures. The card-playing sense is said to be "apparently" from German melden "make known, announce," from Old High German meldon, from Proto-Germanic *meldojan (source of Old English meldian "to declare, tell, display, proclaim"), and the notion is of "declaring" the combination of cards. Related: Melded; melding.