To string up is to hang somebody, or to hang something between two points. String up the lights.
The lights sense is everyday and the other is deliberately brutal. To string up bunting across a street is the commonest use anybody now makes of the phrase.
1phrasestring upTo hang somebody, or to hang something between two points.
“They would string up the lights along the whole of the front.”
string up is a phrase.
Practised in A Song of Ice and Fire