This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
栿 combines 木 (tree/wood) with 伏, which likely contributes a phonetic element. The exact historical development is uncertain.
With 木 (tree) on the left and 伏 (bend down) on the right, picture a wooden beam bending under weight, though its actual meaning is unclear.
For フク, imagine a wooden beam that looks like it might buckle: the sound 'fook' (close to フク) suggests the bending wood.