Sūnaitis

Perhaps from lit. sūnáitis „sūnaus ar dukters vaikas seneliams, vaikaitis, anūkas“ (LKŽK), although it could also be with the suffix -aitis from pers.n. *Sūnus, which we would associate with lit. sons „vyriškos lyties vaikas savo tėvams“ (ibid.).