The term FROST LINE refers to the point at which FROST forms on the ground. The three crossword solutions BRR (3 letters), IMCOLD (6 letters) and MILESTOGOBEFOREISLEEP (21 letters) allude to the coldness associated with FROST. BRR is an onomatopoeia for the sound of shivering, IMCOLD is a phrase used to express feeling cold, and MILESTOGOBEFOREISLEEP is a reference to the poem 'Stopping by Woods on a Snowy Evening' by Robert FROST.