Green’s Dictionary of Slang

(US) abbr. fellow, as a term of address.

1866		H.L. Williams Gay Life in N.Y. 8: Well, you see, here we are, old fel!
1873		J.S. William Old Times in West Tennessee 98: Taking hold of one of his great paws, he remarked: ‘Old fel, you have made your last run’.
1883		Sweet & Knox On a Mexican Mustang, Through Texas 352: ‘Why, ole fel,’ responded one of the youths.