c. 1300, semblaunce, "fact of appearing to view," from Old French semblance "likeness, appearance," from semblant, present participle of sembler "to seem, appear," from Latin simulare "to resemble, imitate," from similis "like, resembling, of the same kind" (see similar).
The meaning "person's appearance, expression, or demeanor" is attested from c. 1400, also "outward appearance, shape, form" (late 14c.), hence, in sense developments perhaps first in Old French, the meanings "mere appearance, show" (early 14c.), and "false, assumed, or deceiving appearance" (1590s). The meaning "person or thing that resembles another" is attested from 1510s. Related: Semblant.