Perseverance

Met a mate for a pint at lunchtime - very nice it was too. Not for the first time, the crests on the lamp posts at the Shore caught my eye. As well as "persevere", the traditional Leith motto, they feature: sigillum oppidi de Leith.

Intrigued by this, I googled the phrase when I got back to the office. No results. I tried searching for the words individually, and got one result - a foreign language page that wouldn't open. Finally, I put it into an online Latin translator which gave "seal fortified town about Leith" as the meaning.

Two possibilities occur to me: a) the phrase is so ancient, all other references to it have been lost in the mists of time, or b) it's a bit of fake Latin, cobbled together by the council to make Leith seem more posh. Anyone know for sure?