mell v.

early 14c., "to mingle, blend, mix" (a sense now obsolete), from Old North French medler (Old French mesler, 12c., Modern French mêler) "to mix, mingle, to meddle," from Vulgar Latin *misculare (source of Provençal mesclar, Spanish mezclar, Italian mescolare, meschiare), from Latin miscere "to mix" (from PIE root *meik- "to mix").

From late 14c. as "busy oneself, be concerned with, engage in," and in the disparaging sense of "interfere or take part in inappropriately or impertinently, be officious, make a nuisance of oneself" (the notion is of meddling too much), which is the surviving sense of the word. From mid-14c. to c. 1700 it also was a euphemism for "have sexual intercourse." Related: Meddled; meddling.

"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 *meldojanan (source of Old English meldian "to declare, tell, display, proclaim"), and the notion is of "declaring" the combination of cards. Related: Melded; melding.