In a certain code language, 'it pit sit' means 'I am boy', 'it nit sit' means…

Concept: In word-comparison coding-decoding puzzles, when two coded phrases share code words, those shared words must stand for whatever meaning is common to the two decoded sentences; the code word that differs between the phrases must correspond to the meaning that differs between the sentences.

Application: Comparing 'it pit sit' (= 'I am boy') with 'it nit sit' (= 'I am girl'): the words 'it' and 'sit' occur in both coded phrases, and 'I' and 'am' are the words common to both decoded sentences, so 'it' and 'sit' together code the shared meaning 'I' and 'am'. 'pit' occurs only in the first phrase, which decodes to 'I am boy', so 'pit' codes 'boy'. 'nit' occurs only in the second phrase, which decodes to 'I am girl', so 'nit' codes 'girl'.

Cross-check: Substituting back — 'it', 'pit', 'sit' as 'I', 'am', 'boy' in some order reads 'I am boy', and 'it', 'nit', 'sit' as 'I', 'am', 'girl' reads 'I am girl' — both consistent, confirming 'nit' means 'girl'.