Moreover; in addition to.
This term has nothing to do with footwear. The 'boot' is thought to be a derivative of the earlier 'bat' meaning 'good or useful'. This is also the root of the word 'better'.
Forms of 'to boot' in Old English date from around 1000AD. Robert Manning of Brunne included a version of it in Langtoft's Chronicle, 1330:
"A hundreth knyghtes mo... and four hundreth to bote, squieres of gode aray."