The word 'thirdly' is a transparent formation from the ordinal number 'third' and the ubiquitous adverbial suffix '-ly'. The root 'third' traces back to Proto-Indo-European tritiyos, meaning "third", which evolved through Proto-Germanic into Old English þridda. The suffix '-ly' originates from the Old English -līċe, meaning "having the form or body of", which was used to create adverbs from adjectives. The logical construction is perfectly straightforward: the concept of "third" is combined with the grammatical marker for "in the manner of" to create a sequencing adverb meaning "in the third place". This pattern is consistent with other sequence adverbs like 'firstly' and 'secondly', forming a clear and systematic set for enumerating points in discourse.

Structure: third (three) + ly (adverb suffix) - third: From Old English þridda, from Proto-Germanic þridjô, from Proto-Indo-European tritiyos. Functions as the base numeral. - ly: From Old English -līċe, an adverbial suffix. Functions as a derivational suffix forming adverbs.