The word is rare in French: Godefroy provides one attestation from Wace’s Roman de Rou ( for osteor, glossed ‘combattant, guerrier, militaire’) and another from Froissart’s Cronicle (sub ostoier 2, glossed ‘soldat’). The DMF and T/L do not add any further attestations. Although the word is not found in medieval English, the OED has the comparable entry hoster n.2, glossed as ‘one who serves in a host of army’, with an earliest attestation of 1892.

Alternatively – and without further context – the word may also be interpreted as a variant of the formally similar but semantically unrelated oster² (‘hosteler, landlord of an inn’), which is equally rare (and just as problematic to gloss).