drug (n.)youdaoicibaDictYouDict
late 14c. (early 14c. in Anglo-French), "medicine, chemical ingredients," from Old French droge "supply, stock, provision" (14c.), which is of unknown origin, perhaps from Middle Dutch or Middle Low German droge-vate "dry barrels," or droge waere, literally "dry wares," but specifically drugs and spices, with first element mistaken as word for the contents (see dry goods), or because medicines mostly consisted of dried herbs.

Compare Latin species, in Late Latin "wares," then specialized to "spices" (French épice, English spice). The same source produced Italian and Spanish droga, Swedish drog.

Application to "narcotics and opiates" is late 19c., though association with "poisons" is 1500s. Druggie first recorded 1968. To be a drug on or in the market (mid-17c.) is of doubtful connection and may be a different word, perhaps a play on drag, which was sometimes drug c. 1240-1800.
flaky (adj.)youdaoicibaDictYouDict
1570s, "consisting of flakes," from flake + -y (2). Meaning "eccentric, crazy" first recorded 1959, said to be American English baseball slang, but probably from earlier druggie slang flake "cocaine" (1920s). Flake (n.) "eccentric person" is a 1968 back-formation from it. Related: Flakiness.
The term 'flake' needs explanation. It's an insider's word, used throughout baseball, usually as an adjective; someone is considered 'flaky.' It does not mean anything so crude as 'crazy,' but it's well beyond 'screwball' and far off to the side of 'eccentric.' ["New York Times," April 26, 1964]