I dunno what his origin is already in the comics cuz I doubt anyone gives a shit about the origin story of Absorbing Man, but my interpretation is that he was actually an attraction in PT Barnums “Greatest Show On Earth” before he was eventually captured by a recently-established S.H.I.E.L.D due to going on a rampage and getting in a fight with a mysterious assailant who was never found afterwards. He also has partial immortality (similar to The Thing) which allows him to live and become a prominent threat in the modern day.