[1] Despite its rarity, the word has occasioned quite extensive scholarly debate because it may give insight into Old Nordic religion.
The first attestation is in a rather cryptic kenning in stanza 10 of the skaldic poem Glælognskviða by Þórarinn loftunga, thought to date from 1030×34.
One of the exhortations to Sveinn to pray runs which appears literally to mean 'when you perform/present your prayers in front of the sacred nail(s) [reginnagla] of the language/speech/measure/inlaid decoration of books'.
[2] The main interpretations of the phrase 'reginnagla bóka máls' have been: Of these, 'Olaf' has historically been the most common and 'liturgical book', suggested by Margaret Clunies Ross, the most recent (as of 2014).
The other attestation of reginnaglar is in the Icelandic saga Eyrbyggja saga, which relates the use of reginnaglar in the construction of a temple by Þórólfur Mostrarskegg (Thorolf Most-Beard): Here, the nails clearly represent some kind of metal, nail-like decorative feature of the high-seat pillars, and Clunies Ross sees it as plausible that despite the lateness of the source, it does represent a feature of pre-Christian material culture.