閾下 is read as いきか (ikika) and means subliminal.
いきか
ikika
いきちか
subliminal
いきち
threshold (amount, dose, etc.)
てんか
shogun (Edo period)
いか
including ...
ていか
fall
ろうか
corridor
ぶか
subordinate person
ちか
underground
でんか
your Highness
めした
subordinate
じょうげ
first and second volumes
らっか