Top cave/hole radical - the indexing radical. Frames 窃 as something done in a hidden burrow, the classic image of a thief sneaking into a cavity. Sits over the family of hole-words: 空 (empty), 突 (sudden), 窗 (window), 究 (investigate).
Bottom 切 supplies the sound (qiē → qiè, tone change) and a faint cutting/slicing nuance fitting petty theft. This phonetic was substituted in the 1956 reform - traditional 竊 used a far denser bottom. Same qiē sound key in 砌 (qì) and 沏 (qī).
