Unknown. Possibly borrowed from Proto-Germanic *stīfaz (~ *stīfijaz?) (“stiff, rigid”) (compare other Germanic borrowings like tyyris < *diurijaz, raitis < *raidi(ja)z), implying Proto-Finnic *tiibis ~ *tiivis (without any Finnic cognates). In this case, the original sense would have been “tight, compact, solid”. However, a gradation pattern like v ~ p (with a genitive form like **tiipiin) would be expected, but such is not attested. It is possible that the gradation was levelled through the verb tiivistää, through the seemingly back-formed forms tiivi, tivi found in some dialects, or by later influence from e.g. Middle Low German stīf.