In a cruising manner; while cruising, or in the manner of a cruise; slowly and leisurely.
From 'cruise' (Dutch 'kruisen') with '-ing' forming the present participle, then '-ly' added to create an adverb—a three-step word formation showing English productivity.
This is a 'double adverb' formation that's grammatically valid but rarely used—most speakers would say 'while cruising' instead, showing how language favors simpler structures.
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