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.