inrush is a modern English compound of in (adverb) + rush (the verb/noun), the latter from Anglo-Norman russher / Old French ruser ('to drive back, repel'), of uncertain ultimate origin. Morphologically, the word is best read through the components named in the history, with the modern spelling preserving a compound structure rather than a single unanalyzable stem. The etymology remains cautious where the history is cautious: comparisons, echoic formation, or disputed derivation do not make the ancestry more definite than the forms actually named. In present English, the noun sense is "a sudden rushing in or influx", so the historical development ends in a clearly identifiable modern dictionary use. The same base also appears in related forms such as rush, onrush, and outrush. Its semantic neighborhood includes influx, inflow, inflood, and surge, which helps locate the modern sense without adding a separate etymological claim. The named historical forms include Old English in.