slender adj.

c. 1400, "thin, lean, not fat or fleshy," earlier sclendre (late 14c.), Anglo-French esclendre, from Old French esclendre "thin, slender," which could be from Old Dutch slinder, but according to OED the connection is doubtful.

From 1510s of things, "small in width or diameter as compared to length." By 1520s as "weak, feeble, slight, insignificant, trifling." Related: Slenderly; slenderness.