窦 (dòu) is a bound form (b.f.) — it does not appear alone but in compound words. Its core meaning is 'hole, cavity, or small passage', as in 鼻窦 (bí dòu, 'nasal sinus') or 疑窦 (yí dòu, 'doubt, suspicion'). It is also a common surname (e.g., 窦先生). Because it is a bound form, you will never see 窦 standing by itself in modern Chinese; it always attaches to another character.