Lewis & Short

3. -ăn. This word appears in forsan, forsitan, and fortasse an (Att. Trag. Rel. p. 151 Rib.) or fortassan, seeming to enhance the idea of uncertainty and doubt belonging to fors, etc., and is regarded by some as the Greek conditional particle ἄν, and indeed one of these compounds, forsitan, sometimes in the Vulgate, translates ἄν; as, Joan. 4, 10; 5, 46; 8, 19; and in 3, Joan. 9, it still represents the various reading, ἄν.