what's Sindarin for "shut up" or "be quiet"?

[Sharkû]

As per Quendi and Eldar, HoME XI, 4:

4(a) ego! 'be off!' This is from *hek(e) a: a the imperative
particle, being originally independent and variable in place,
survived in S as o > o, though this now always followed the
verb stem and had become an inflexion.
4(b) eitha-. This is in the main a derivative of PQ *ek-ta,
and means 'prick with a sharp point', 'stab'; but the sense
'treat with scorn, insult' (often with reference to rejection or
dismissal) may show the effect of blending with PQ *hek-ta.
To say to anyone ego! was indeed the gravest eithad.