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fin 1 (fĭn)
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n.
1. A membranous appendage extending from the body of a fish or other aquatic animal, used for propelling, steering, or balancing the body in the water.
2. Something resembling a fin in shape or function, as:
a. A covering for the foot having a flat flexible portion made of rubber or plastic that widens as it extends forward from the toes, used to provide enhanced propulsion in swimming and diving.Also called flipper.
b. A fixed or movable airfoil used to stabilize an aircraft, missile, or projectile in flight.
c. A thin, usually curved projection attached to the rear bottom of a surfboard for stability.
d. A projecting vane used for cooling, as on a radiator or an engine cylinder.
e. See tail fin.
v.finned, fin·ning, fins
v.tr.
To equip with fins.
v.intr.
1. To emerge with the fins above water.
2. To swim, as a fish.
3. To lash the water with the fins. Used of a dying whale.

[Middle English, fromOld Englishfinn.]
(click for a larger image)
(click for a larger image)
fin1
top: fins of a typical bony fish
bottom: swimming fins

The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2017 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
 
fin 2 (fĭn)
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n.
Slang
A five-dollar bill.

[Yiddish finf, five, from Old High German funf, finf; see penkwe in the Appendix of Indo-European roots.]

The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2017 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
 
Fin.
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abbr.
1. Finland
2. Finnish

The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2017 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
 

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