I must dash
What it really means
A polite way to leave: "I have to go now". Dash means run. It may be true, or just a kind way to end a long conversation. No offence is meant.
Context, tips & common mistakes
"Must dash" — with 'I'd better let you go', 'right, I'll let you get on', and the mighty 'anyway…' — belongs to the toolkit of polite escape: phrases that end conversations while pretending the ending is regrettable or even for the other person's benefit. 'I'll let you get on' is particularly elegant, framing your escape as courtesy to them.
Listen for 'anyway' with a slight change of tone: it is the universal pre-departure signal, and continuing a long story past someone's third 'anyway' is a genuine social error. Your own exits should use the same machinery — an abrupt 'I'm going now' sounds cold in a culture where departures are choreographed. Note that leaving a gathering involves announcing it well before actually leaving; the gap between 'right, we should head off' and the door closing is traditionally substantial.