'rectally' originates from English adjective 'rectal', ultimately from Latin. 'rectal' comes from Medieval Latin 'rectalis', from Latin 'rectus' meaning 'straight' and the neuter 'rectum' literally 'a straight [thing]'; the adverb was formed in English by adding the suffix '-ly' to 'rectal'.

'rectally' changed from Latin 'rectus'/'rectum' into Medieval Latin 'rectalis', then into English as the adjective 'rectal', and finally into the adverb 'rectally' through regular -ly derivation.

Initially related to the idea 'straight' (Latin 'rectus'), the term became an anatomical name 'rectum' for the straight part of the large intestine; it later acquired adjectival meaning 'relating to the rectum' and the adverbial sense 'by way of the rectum' (current meaning).