Welter can be used both as a noun (meaning "turmoil" or "chaos") and a verb. The verb is the older of the two; it has been part of English since at least the 1300s, while the earliest uses of the noun date from the late 1590s. Both noun and verb have roots related to Dutch and Germanic terms meaning "to roll," and both have found a place in historical English literature. The verb helps demonstrate extreme despair in the early Arthurian legend Morte Arthure ("He welterys, he wristeles, he wrynges hys handes!"), and in 1837 Thomas Carlyle used the noun in The French Revolution ("I leave the whole business in a frightful welter: … not one of them understands anything of government").
Middle English welteren, weltryn "to turn over, tumble, writhe, take unrestrained pleasure (in)," frequentative derivative of welten "to topple, overturn, fall over," by-form (perhaps from a Germanic weak verb *waltjan-) of walten "to turn over, upend, be overturned, cast, throw, surge," going back to Old English -wæltan (in gewæltan "to roll"), going back to a Germanic verbal base *walt-, *welt- "roll," found in a variety of attested formations (as Old English awyltan "to roll away," unwealt "steady," Middle High German walzen "to roll over," Old Icelandic velta [strong verb, intransitive] "to roll, roll over," velta [transitive] "to set rolling," Gothic waltjan "to surge against [of waves]," uswaltjan "to overturn"), going back to Indo-European *u̯el-d-, extended form of *u̯el(H)- "roll," whence, with various vowel grades and stem formations, Old Irish fillid "(s/he) bends, turns back" (< *u̯el-n-), Old Church Slavic valiti sę "to roll (intransitive)," Lithuanian veliù, vélti "to full (cloth), roll," Greek eiléō, eileîn "wind, turn round, roll up" (< *u̯el-né-), íllō, íllein in same sense (< *u̯i-u̯l-ō), Armenian glem "to roll"
Note: The Middle English verb is paralleled by Middle Dutch welteren and Middle Low German weltern, which Oxford English Dictionary, first edition, regards as the source of the English word. — Lexikon der indogermanischen Verben, 2. Auflage (Wiesbaden, 2001), enters two etyma, *u̯el- "to turn, roll" (drehen, rollen) and *u̯elH- "to roll, seethe" (wälzen, wallen), presumably on the grounds that Lithuanian vélti, with acute intonation, would suggest a laryngeal, while there is no suggestion of a laryngeal in Greek eiléō, etc. For present purposes, etyma with the meaning "seethe, bubble" are treated separately, under well entry 2. Also treated under *u̯el- in the Lexikon are verbs showing an extension with a semi-vowel, *u̯el-u̯-, which are covered here at wallow entry 1. Additionally, there are stems ending in a velar, *u̯el-k-/*u̯el-gh- "to roll"; these are covered here at walk entry 1. All of these elements, as well as many nominal formations, are treated as extensions of a single root *u̯el- in J. Pokorny, Indogermanisches etymologisches Wörterbuch.