قاب: مَسَافَة (قَصِيرَة) (short) distance, (small) space

قاب (القَوْسِ) space between the middle and the end of a bow

على قابِ قَوْسَيْن (مِنْ) at two bow's length (from) , within an ace (of) , very close (to) , very near (to) , on the point (of) , on the verge (of)