and (conj.) Look up and at Dictionary.com
Old English and, ond, originally meaning "thereupon, next," from Proto-Germanic *unda (source also of Old Saxon endi, Old Frisian anda, Middle Dutch ende, Old High German enti, German und, Old Norse enn), from PIE root *en "in."

Introductory use (implying connection to something previous) was in Old English. To represent vulgar or colloquial pronunciation often written an', 'n'. Phrase and how as an exclamation of emphatic agreement dates from early 1900s.