A member of the Congregation of the Assumption, a Roman Catholic religious order, or someone who makes or relies heavily on assumptions.
From 'Assumption' (religious concept of Mary's bodily assumption into heaven) plus '-ist' (follower or practitioner). The Assumptionists are an actual Catholic religious community founded in the 19th century, but the word also generically means someone prone to assuming things.
The Assumptionists were founded in 1845 and became famous for educational missions, but linguistically the word 'assumptionist' for 'someone who assumes' predates the religious order—it's one of those rare cases where a specific proper noun (a religious community) shares a word with a general descriptive term!
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