This rather elegant way of referring to someone as "fat" is, with one exception, not recorded in English until the 1720s. When the word first arrived, in the early 16th century, it meant "stately, majestic, dignified". This is the only possible sense if applied to ships, as when Salerio describes Antonio's argosies as having "portly sail" (Merchant of Venice I.i.9), or to abstract nouns, as when Worcester talks of "greatness" as being "portly" (Henry IV Part 1, I.iii.13). The exception is Falstaff's description of himself as having a "portly belly" (Merry Wives of Windsor, I.iii.57). Achilles, too, is described as being of "large and portly size" (Troilus and Cressida, IV.v.162). These are unusual uses: it is the sense of "dignity" which Shakespeare usually uses. When Capulet tells Tybalt that Romeo "bears him like a portly gentleman" (Romeo and Juliet, I.v.66), he is not suggesting that the great lover is overweight.
David Crystal is the author, with Ben Crystal, of Shakespeare's Words, published by Penguin