I am not a scholar of the classics, so I may be wrong, but there was a trend in England to give a child’s position in the ‘litter’ as their middle name, in Latin. E.g. I have a friend whose middle name is “Quintus” (fifth-born), and there are many people from previous centuries called “Septimus” (seventh-born). I wonder if Triestengestus means third-born, thirteenth-born (or even 30th-born??!). Or maybe it means that the person in question was born in Trieste? Can anyone confirm/refute this?
My first thought was that this could be a dog-latin joke indicating that the child concerned was either conceived or born in Trieste.
However, given the political conditions in Europe in 1916, it seems very unlikely that an Englishman could have been conceived or born in (what was then) Austria.