Difficult to understand; obscure, recondite, or overly complicated in a way that makes it hard to grasp.
From Latin abstrusus 'hidden, concealed' (past participle of abstrudere: ab- 'away' + trudere 'to thrust'). Related to abstruse but appears as a variant spelling in older texts.
This is basically an archaic spelling of 'abstruse'—philologists find these variant spellings fascinating because they show how spelling wasn't standardized in English until the 1700s-1800s!
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