This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
薭 combines 艹 (grass) with 稗, which itself contains 禾 (grain) and 卑. The exact historical development is uncertain, but it likely refers to a type of grass or grain plant.
The top grass radical 艹 over 稗 (a type of barnyard grass) suggests a plant related to grass or grain, but its specific meaning is unclear.
For ハイ, imagine a field of tall grass where you say 'hi' (ハイ) to a hidden scarecrow: ハイ sounds like 'hi', and the grass hides the figure.