late 15c., portli, "stately, dignified, of noble appearance and carriage," from port (n.3) "bearing, carriage" + -ly (1). Meaning "stout, somewhat large and unwieldy in person" is attested by 1590s.
Entries linking to portly
port n.3
"bearing, mien, carriage, demeanor, deportment," c. 1300, from Old French port "carriage demeanor," from porter "to carry," from Latin portare "to carry" (source of port (v.2)), from PIE root *per- (2) "to lead, pass over." From c. 1400 as "external appearance;" by 1520s in the now-archaic sense of "state, style, establishment."
-ly 1
suffix forming adjectives from nouns and meaning "having qualities of, of the form or nature of" (manly, lordly), "appropriate to, fitting, suited to" (bodily, earthly, daily); irregularly descended from Old English -lic, from Proto-Germanic *-liko- (Old Frisian -lik, Dutch -lijk, Old High German -lih, German -lich, Old Norse -ligr), related to *likom- "appearance, form" (Old English lich "corpse, body;" see lich, which is a cognate; see also like (adj.), with which it is identical).