The word is attested only once in A-N, and appears in TLL without further context (as a gloss to natrix in John of Garland’s Latin Unus Omnium – a text which remains unpublished as a whole). The DMLBS’s entry for natrix defines the word as ‘a. snake, b. watersnake, c. blindworm and d. serpent’ (1889c). The Unus Omnium attestation is not included in the article.
However, the A-N word, appearing next to the words for ‘to swim’ and ‘swimmer’ suggests the sense ‘female swimmer’. The Latin for female swimmer would normally be natatrix (DMLBS 1888b), but its masculine counterpart natator (DMLBS 1888b) is attested in a similarly contracted form nator (DMLBS 1889c). Both of the possible senses (‘female swimmer’ vs. ‘water snake’) are supported by Gdf, TL and DMF. As a result and without any further context it is impossible to determine which applies to this particular citation.