stand on tiptoes
跂 centers on the action of standing on tiptoes, stretching upward to reach or see.
跂 combines 足 (foot) with 支, which likely contributes the sound and suggests supporting or propping up, giving the idea of standing on tiptoes.
The left side 足 means foot, and the right side 支 looks like a prop or support. Picture a foot propped up on its toes, stretching upward.
For キ, imagine standing on tiptoes to reach a key on a high shelf: key -> キ, and your foot stretches up to get it.