You can replace the dashes with the same four-letter word, and create two new words, one that starts with ARM, and one that ends in ING.
Can you find all possible four-letter words that work?
arm----
----ing
Puzzle Copyright © Kevin Stone
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Hint
There are 4 possible answers.
Answer
Band, load, lock, and rest, all create new words:
armband and banding
armload and loading
armlock and locking
armrest and resting
Note
Some dictionaries also allow ARMRACK, but others don't. So RACK is a unofficial possible answer.