Compositional etymology. It must be compared to the one provided for λάτρον "wages" (see λάτρον / λα- + τηρέω). Next to the derivational etymology deriving λάτρις from λάτρον, the two compositional etymologies share the identification of λα- as the intensive prefix but differ  on the second member. While λάτρον "wages" is assumed to be a compound of τηρέω "to watch over", λάτρις "hired man, servant" is supposed to be a compound of τρέω "to be frightened", meaning "he who fears much". It is likely that the first etymology is that of λάτρις, which echoes that of other names of the slave as "fearing" (see δοῦλος / δέω + ὅλος). But obviously it could not fit for λάτρον, an inanimate noun. Therefore the etymology λα- + τηρέω was designed: it is less close formally, because it implies a syncope, whereas the etymology λα- + τρέω for λάτρις does not.