This study distinguishes diverse interactional uses of ICH WEIß NICHT (‘I don’t know’) and debates their link to dissimilarity in argument structure (SV(O), (O)VS, V-only). After variants with full complement, speakers underscore their knowledge deficit or show hesitation to reply. After variants without object complement, speakers show insecurity about the verity of the ensuing premise or about its adequacy as reply, operating as a prepositioned epistemic hedge or a pragmatic marker molding the ensuing TCU. When ICH WEIß NICHT is employed as reply to a assertion, it indexes disagreement (separately from all completion patterns).