press, print, affix a seal, stamp
捺 centers on applying downward pressure to leave a mark or make something adhere: pressing, stamping, or affixing a seal. The print sense is a natural extension of pressing an inked surface onto paper.
捺 combines 扌 (hand) with 奈, which likely contributes the sound ナツ. The character expresses the action of pressing or stamping with the hand.
A hand 扌 presses down on 奈, like a seal being firmly pushed onto a document. The pressure leaves a clear stamp or print.
For ナツ, imagine pressing a seal onto a document during a hot summer day: 夏 (なつ, summer) -> ナツ, and the heat makes the stamp stick firmly.