Bolster

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Old English, of Germanic origin; related to Dutch bolster and German Polster .

wiktionary

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From Middle English bolster, bolstre, from Old English bolster(“pillow”), from Proto-West Germanic *bolstr, from Proto-Germanic *bulstraz(“pillow, cushion”). Cognate with Scots bowster(“bolster”), West Frisian bulster(“mattress”), Dutch bolster(“husk, shell”), German Polster(“bolster, pillow, pad”), Swedish bolster(“soft mattress, bolster”), Icelandic bólstur(“pillow”).

etymonline

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bolster (n.)

Old English bolster "bolster, cushion, something stuffed so that it swells up," especially "a long, stuffed pillow," from Proto-Germanic *bolkhstraz (source also of Old Norse bolstr, Danish, Swedish, Dutch bolster, German polster), from PIE *bhelgh- "to swell," extended form of root *bhel- (2) "to blow, swell." Applied since 15c. to various parts which support others.

bolster (v.)

mid-15c. (implied in bolstered), "prop up; make 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.