。
Calm down, would you.
Literal
[stem command-落ちつき][soft imperative-な].
~な appended to a masu-stem (落ち着き → 落ち着きな) is a casual mild imperative — softer than the abrupt 落ち着け. Common in colloquial speech, particularly from older speakers, parents, or in Tokyo dialect. Crucially distinct from the negative imperative ~な, which attaches to dictionary form (落ち着くな = 'don't calm down').