The editor of Five Med MSS suggests ‘perhaps a fermenting mixture’ as derived from the Latin verb surgere: ‘to rise’ (glossary, p. 248), which seems unlikely. Alernatively, the word may be related to Latin sorda, DuCange 6.301b: 'coating of lime' ('Litura ex calce composita'). In the absence of convincing etymological evidence, any definition remains conjectural.