想当然 is a fixed verb phrase (set phrase) meaning 'to take something for granted' or 'to assume without questioning'. It describes a thoughtless or automatic assumption, often used critically to point out a lack of careful consideration. You can use it as a predicate or with 地 as an adverb (想当然地认为). Literally 'think of course', but the actual meaning is broader — it's not just thinking something is obvious, but assuming without evidence.