The name Ratatoskr contains two elements: rata- and -toskr. The element toskr is generally held to mean "tusk". Guðbrandur Vigfússon theorized that the rati- element means "the traveller". He says that the name of the legendary drill Rati may feature the same term. According to Vigfússon, Ratatoskr means "tusk the traveller" or "the climber tusk."
Sophus Bugge theorized that the name Ratatoskr is a loanword from Old English meaning "Rat-tooth." Bugge's basis hinges on the fact that the -toskr element of the compound does not appear anywhere else in Old Norse. Bugge proposed that the -toskr element is a reformation of the Old English word tūsc (Old Frisian tusk) and, in turn, that the element Rata- represents Old English ræt ("rat").
According to Albert Sturtevant, "[as] far as the element Rata- is concerned, Bugge's hypothesis has no valid foundation in view of the fact that the [Old Norse] word Rata (gen. form of Rati*) is used in Háv[amál] (106, 1) to signify the instrument which Odin employed for boring his way through the rocks in quest of the poet's mead [...]" and that "Rati* must then be considered a native [Old Norse] word meaning "The Borer, Gnawer" [...]".
Sturtevant says that Bugge's theory regarding the element -toskr may appear to be supported by the fact that the word does not appear elsewhere in Old Norse. Sturtevant, however, disagrees. Sturtevant says that the Old Norse proper name Tunne (derived from Proto-Norse *Tunþē) refers to "a person who is characterized as having some peculiar sort of tooth" and theorizes a Proto-Germanic form of -toskr. Sturtevant concludes that "the fact that the [Old Norse] word occurs only in the name Rata-toskr is no valid evidence against this assumption, for there are many [Old Norse] hapax legomena of native origin, as is attested by the equivalents in the Mod[ern] Scandinavian dialects." Modern scholars have accepted this etymology, listing the name Ratatoskr as meaning "drill-tooth" (Jesse Byock, Andy Orchard, Rudolf Simek) or "bore-tooth" (John Lindow).