fin
鰭 centers on the fin of a fish or other aquatic animal.
鰭 combines 魚 (fish) with 耆, which likely contributes the sound and may relate to an older idea of a projecting part. The exact historical development of 耆 in this character is uncertain.
The left side is a fish 魚, and the right side 耆 looks like an old man with a long beard. Picture an old fish with a prominent, flowing fin like a beard.
For キ, imagine a fish with a fin shaped like a key: key -> キ, and the key-fin unlocks the fish's swimming power.
fin (e.g. of a fish)
caudal fin; tail fin
dorsal fin
mid-sized konoshiro gizzard shad (i.e. about 15 cm in size)
Wilson's phalarope (Phalaropus tricolor)
short-finned pilot whale (Globicephala macrorhyncus)
bones supporting the fins of a fish
Actinopterygii (class of the ray-finned fishes)
red phalarope; grey phalarope (Phalaropus lobatus)
sailfin poacher (Podothecus sachi)
making something strong even stronger; (giving) fins to a lion
Sarcopterygii (class comprising the lobe-finned fishes)
sarcopterygians (fishes of subclass Sarcopterygii)
pectoral fin
adipose fin
pelvic fin; ventral fin
anal fin
red-necked phalarope (Phalaropus lobatus)
white cloud mountain minnow (Tanichthys albonubes); Canton danio
fin ray