An Etymological Dictionary of the German Language/Annotated/Polster
Polster, masculine and neuter ‘cushion, bolster,’ from the equivalent Middle High German polster, bolster, Old High German bolstar. masculine; corresp. to Dutch bolster, Anglo-Saxon and English bolster, and its equivalent Old Icelandic bólstr. The cognates are connected with the Old Teutonic root belg, ‘to swell,’ to which Balg also belongs, hence the original sense is ‘swelling’ (Teutonic bolstra-, from bolhstro-). With the corresponding Sanscrit root bṛh, ‘to be great.’ Sanscrit upabarhaṇa, ‘bolster,’ is connected; compare also Prussian pobalso, balsinis, ‘pillow,’ Servian blazina (from *bolzina), ‘pillow.’