This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 愂 is uncertain. It visually combines 勃 and 心, but the original component relationship is unclear.
The top 勃 suggests sudden movement or vigor, and the bottom 心 is heart. Imagine a sudden surge of feeling, but the exact meaning is unclear.
For ハイ, picture a sudden burst of energy making you say 'Hi!' with vigor: Hi -> ハイ, as the heart races.