lee of an island; area hidden by an island
Refers to the sheltered side of an island, or a sea area that is hidden from view or wind by an island.
The boat went into the lee of the island to shelter from the wind.
島陰から突然別の船が現れた。
Another boat suddenly appeared from behind the island.
Compound of 島 (shima, 'island') and 陰 (kage, 'shade; hidden area'). The reading しまかげ is a straightforward combination.