An Etymological Dictionary of the German Language/-tum
-tum, suffix, from MidHG. and OHG. -tuom; an abstract suffix, which has been formed in compounds from an independent word tuom, m. and n., ‘relation, rank, dignity, condition.’ This is an abstract form of ModHG. thun. Comp. the corresponding Eng. suffix -dom in kingdom. Eng. has preserved the independent word doom (AS. dôm), to which to deem (AS. dêman) is allied; comp. Sans. dhâman, ‘institution, sacred custom.’