This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
虙 combines the tiger-head enclosure 虍 with 必 inside, but the exact historical development and original meaning are uncertain.
The tiger-head enclosure 虍 surrounds 必 (certainly), but without a clear meaning, picture a tiger hiding something it must protect—though the real meaning is unknown.
For フク, imagine a tiger in a hooded cloak (フク) hiding inside the enclosure: fuku -> フク.
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