壱
イチ, イツ, ひとつ, い, かず

I, one

JLPT1 G7 S7 F2351

The relevant seal inscription form is 壺 lidded pot + 吉# (filled container) → full pot/jar → *one* (← unity of contents). As with 弐, 参/參, and 拾, 壱 is used as a substitute for the usual number (in this case, 一) in documents as a precaution against forgery.

[Full Kanji Etymology List]