This rare word seems to be a tautological construction of quint followed by a superfluous -isme suffix, possibly under the influence of the synonymous cinckime. It is not entirely clear whether this word is genuine. Studer and Evans suggest in the glossary of Lapid that it may be a deviant reading (or perhaps even misreading) of quinzesme, meaning ‘fifteenth’ – a reading which would be supported by the corresponding passage in the First Prose Lapidary, which reads ‘et quant ele est prime, dunc duret la poesté tute jur, e altresi quant ele est quinzieme’ Lapid 106.xxix.6.; cf. quinzime. TL, however, rejects this particular interpretation (‘im Glossar unrichtig’) and links the word with Gdf 6,519a quintième.

In the case of the two Foedera attestations, iii 827 may be read as ‘before the fifth day of May’, but it must be noted that expression le quinzime jur, to indicate the period of a fortnight, seems to be quite common in documents such as these, raising the possibility that this citation belongs under quinzime. In the case of i 879, however, the word can only be interpreted as the ordinal ‘fifth’.