In a costly manner; at high cost or with great expenditure of money.
From 'expensive' (which itself comes from 'expense' plus the adjective-forming suffix '-ive') plus the adverbial suffix '-ly.' This shows a three-layer word structure: Latin root + adjective suffix + adverb suffix.
We use 'expensively' all the time—'She dressed expensively'—but most people don't realize it contains three layers of word-building. It's like how a wooden cabinet contains multiple pieces of wood assembled together.
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