Lewis & Short

frātercŭlus, i, m. dim. [frater], a little brother.

  1. I. Lit.: unde fit, ut malim fraterculus esse Gigantis, i. e. of unknown parents. qs. sprung from the earth, Juv. 4, 98.
  2. II. Transf. (acc. to frater, II. A.), a familiar appellation given to a friend: quam copiose laudatur Apronius a TimarchideVolo, mi frater, fraterculo tuo credas, Cic. Verr. 2, 3, 66, § 155.