"producing an appearance of thinness," 1925, present-participle adjective from slim (v.).
Entries linking to slimming
slim v.
1808 [Jamieson], "to scamp one's work, do carelessly or superficially," from slim (adj.). The meaning "to make slim" (a garment, etc.) is from 1862; that of "reduce (one's) weight" is from 1930. Related: Slimmed; slimming. Adjectival phrase slimmed-down is attested by 1946 in advertisements, from the verbal phrase.