The correct reading ずさん features a special, customary on'yomi (慣用音) ず for 杜, rather than the more common と. 撰 (さん) is a standard on'yomi. Wrong readings like とさん or じゅさん fail to recognize this specific pronunciation. It means sloppy, careless, or slipshod work, often implying unreliability and many errors.

杜 (ず) は慣用音、撰 (さん) は音読み。杜の一般的な音読み「と」に注意。仕事などがいい加減で手落ちが多く、いい加減な状態を指します。