窺いしる[ukagaishiru] to perceive
窺う[ukagau] to peep (through)
窺知[kichi] perception

窺い探る[ukagaisaguru] to spy out
窺く[nozoku] to peek (though a keyhole, gap, etc.)
隙を窺う[sukiwoukagau] to watch for an unguarded moment

隙人[himajin] man of leisure
間隙[kangeki] gap
空隙[kuugeki] vacant space
隙き間[sukima] crevice
手隙[teaki] not busy

手間隙[temahima] time and effort
寸隙[sungeki] spare time
隙間風[sukimakaze] draft
隙かさず[sukasazu] without a moment's delay