From the same as cahar; a dungeon (as surrounded by walls) -- prison.
see HEBREW cahar
H5470. sohar
סֹ֫הַר noun [masculine] roundness (?); — only in הַמּ ׳בֵּית Genesis 39:20 (twice in verse); 39:21-22, 23 (all J), 40:3, 5 (RJ), the round house (name of a prison: but ׳ס perhaps an Egyptian word Hebraized, compare Dr in HastingsDB ii.768 n. and EbÄg. M. 318 f.).