This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 昩 is uncertain. It visually combines 日 (sun) and 末 (end/tip), but the original meaning and component relationship are unclear.
The sun 日 at the end 末 of the day might suggest dusk or fading light, but the actual meaning is uncertain.
For バツ, imagine the sun setting at the end of the day with a sudden 'batsu' sound as it disappears: batsu -> バツ.