KanjiJLPT N2
Armpit

armpit, the other way, another place, flank, supporting role

Mnemonic

Under the moon, a bodybuilder flexes his muscles so hard that sweat erupts from his armpit like a silver geyser. The moonlight catches every droplet streaming from that armpit, making his muscles shimmer as he raises his arms higher, proudly displaying the flood.
Additional thoughts
Picture the classic bodybuilder pose with arms raised high, exposing the armpit. The moon acts as a spotlight, and the exaggerated muscle flex is what forces the sweat out. The combination of moonlit silver and raw physicality locks in the image.
Quick recall
A flexing man's muscles erupt sweat from his armpit under the moon.

Details

The keyword for 脇 is armpit. This kanji refers to the area on the side of the body just beneath where the arm meets the torso. By extension, it can also convey the idea of "side" or "beside" in a more general sense, indicating something that is off to the side or secondary to the main focus. It carries connotations of being peripheral or flanking, much as the armpit flanks the central body.
On'yomi
きょう
Kun'yomi
わき、わけ