Both characters are ancient length units:
咫 was a Zhou-era 'short cubit' of about 8
寸 (~18 cm), and
尺 is the traditional 'foot' (about 23 cm in Han times). Pairing them means 'a short cubit and a foot' — barely a stride. Survives almost only in the idiom
近在咫尺 (jìn zài zhǐchǐ, 'right at hand').
