armpit peek
When a glimpse of a character's armpit is visible through a gap in their clothing. This tag implies that the armpit would not normally be visible except from a particular angle or due to the positioning of the character's body. Good examples of armpit peeks include post #61345 and post #588708.
For images where the character's armpit is uncovered but the inner portion of the armpit isn't visible, e.g. because the arm is lowered or the armpit is turned away from the viewer's perspective, use armpit crease.