笃 is a bound form (b.f.) meaning 'sincere, earnest' — it almost always appears as part of two-character words like 笃定 ('confident, sure'), 笃信 ('to believe firmly'), or 笃学 ('diligent in study'). When used of illness (病笃), it means 'critical' or 'grave'. You won't use 笃 alone as a standalone word; it's a literary root that attaches to other morphemes.