שִׂיאוּר, (v. שְׂאוֹר) leavening; (sub. עיסת) dough beginning to ferment. Pes. III, 5 ש׳ ישרף dough beginning to ferment (on Passover) must be burnt, contrad. to סִירּוּק. Ib. איזהו ש׳ כקרני וכ׳ dough is called siur, when the cracks on the surface spread like the horns of locusts; (anoth. opin.) כל שהכסיפו וכ׳, v. כָּסַף. Men. 53ᵃ (ref. to Lev. VI, 10) ואימא … אלא ש׳ may I not interpret, ‘it shall not be baked leavened’, but it may be baked as siur? Ib. ש׳ דמאן siur as defined by whom (by R. Meir or R. Judah)?; a. e.—Y. Pes. V, 32ᵇ bot. שחטו על סִיעוּר if a person slaughtered the Passover lamb while having in his house dough just beginning to ferment.—V. סִיאוּר.