climb
躋 centers on the physical act of climbing or ascending, typically upward movement.
躋 combines 足 (foot) with 齊, which likely contributes the sound and may suggest evenness or order, together giving the idea of stepping upward in an orderly climb.
The foot radical 足 on the left and the orderly-looking 齊 on the right suggest feet moving in a steady, even climb.
For セイ, imagine climbing a steep hill and saying 'say' with effort: say -> セイ, as you push upward with your feet.