Speech radical on the left, the indexing component and a contracted form of 言. It pegs 诈 firmly as a verbal act - words used to cheat - and groups it with other speech-based deception verbs: 谎 lie, 骗 trick (no 讠 but related), 谣 rumour, 讥 mock.
Right side 乍 supplies the sound exactly - same syllable, same tone. The same phonetic powers a tight zhà family: 炸 explode, 榨 squeeze, 蚱 grasshopper. Once you can read 乍 on sight, the entire family pinyin locks in. Meaning here is purely sound.
