An archaic or poetic form of 'along' or 'alongside,' used in older English texts.
From 'along' plus the archaic suffix '-st' (which also appears in words like 'betwixt,' 'amongst'). This suffix was an older way of forming prepositions, and 'alongst' represents English from approximately the 15th-17th centuries.
Words like 'alongst,' 'amongst,' and 'betwixt' show that Old English loved adding '-st' to make prepositions sound fancy and formal—it's like the ancestors of 'amongst' and 'amidst' that modern English kept but 'alongst' forgot!
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