At arm's length means as far away as your reach, or kept at a distance. Hold it at arm's length, keep him at arm's length.
The figurative use is the commoner one and means deliberately not close. In business an arm's length arrangement is one between parties with no special relationship, which is a term of art.
1phraseat arm's lengthAs far away as your reach, or kept at a distance.
“She held the letter at arm's length to read it.”
at arm's length is a phrase.
Practised in Directions & Position