mid-15c. (implied in bolstered), "propped up, made to bulge" (originally of a woman's breasts), from bolster, n.. Figurative sense is from c.1500, on the notion of "to support with a bolster, prop up." Related: Bolstering.

Old English bolster "bolster, cushion, something stuffed so that it swells up," especially "long, stuffed pillow," from Proto-Germanic *bolkhstraz (cognates: Old Norse bolstr, Danish, Swedish, Dutch bolster, German polster), from PIE *bhelgh- "to swell" (see belly, n.).