notch of an arrow, ought, must, should be, expected
筈 centers on the notch of an arrow, the groove that fits onto the bowstring. The abstract meanings of 'ought', 'must', and 'expected' come from the idea of something fitting exactly as it should, like an arrow nocked in place.
筈 combines 竹 (bamboo) with 舌 (tongue), likely representing a bamboo arrow notch. The phonetic role of 舌 is uncertain, but the character is associated with the notch of an arrow.
The top 竹 suggests bamboo, from which arrows are made, and the bottom 舌 looks like a tongue-shaped notch. Picture a bamboo arrow with a tongue-like groove at the end, fitting perfectly onto the bowstring.
For カツ, imagine an arrow notch that fits so snugly it makes a sharp 'cuts' sound when released: cuts -> カツ, and the arrow flies exactly as expected.
should (be); bound (to be); expected (to be); must (be)
nock of an arrow
only natural; no wonder; as expected; of course; as it should be
herring-bone pattern; herringbone pattern
common vetch (Vicia sativa); tare
it cannot be that ...; it is highly unlikely that ...; it is impossible that ...
it cannot be that ...; it is highly unlikely that ...; it is impossible that ...
that is not supposed to ...; that ought not to ...; that should never ...; that cannot possibly ...
hemp rope fastened from the bow of a Japanese ship to the tip of the mast