die Schelle
slap
Colloquial for a slap in the face; a quick, flat blow to the face with the hand.
Er hat für seine freche Bemerkung eine ordentliche Schelle kassiert.
He got a proper slap for his cheeky remark.

slap / clamp / jingle bell
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slap
Colloquial for a slap in the face; a quick, flat blow to the face with the hand.
Er hat für seine freche Bemerkung eine ordentliche Schelle kassiert.
He got a proper slap for his cheeky remark.

The context makes the meaning clear. Verbs like 'geben' (to give) or 'kriegen' (to get) point to the slap. Technical words like 'Rohr' (pipe) or 'befestigen' (to fasten) indicate the bracket/clamp. Words like 'klingeln' (to jingle/ring) or 'Schlitten' (sleigh) show that the small bell is meant.
Er hat ihm eine Schelle gegeben.
He gave him a slap.
In the colloquial meaning 'Ohrfeige' (slap in the face), the fixed combinations 'jemandem eine Schelle geben' (to give someone a slap) (active) and 'eine Schelle bekommen' or 'kriegen' (to get a slap) (passive) are the most common.
Pass auf, was du sagst, sonst kriegst du gleich eine Schelle.
Watch what you say, or you'll get a slap in a second.
The meaning 'Halterung' (bracket/clamp) is found almost only in technical contexts, for example in hardware stores, in instructions, or in conversations with tradespeople.
Ich brauche noch eine 20-Millimeter-Schelle für das Wasserrohr.
I still need a 20-millimeter clamp for the water pipe.
When the small bell is meant, this is often made clear by verbs of hearing or sound production. Typical verbs are 'klingeln' (to jingle/ring), 'läuten' (to ring), or 'hören' (to hear).
Im Winter hörte man oft das Klingeln der Schellen von den Pferdeschlitten.
In winter, you could often hear the jingling of the bells from the horse-drawn sleighs.
blow / stroke
'der Schlag' and 'die Schelle' can both mean that someone is hit with the hand. 'der Schlag' is general and can refer to many kinds of blows or hits. 'die Schelle' is colloquial and, in this meaning, refers to a slap in the face, that is, a quick hit with the flat hand to the face. So 'Schlag' is more neutral and broader. 'Schelle' only fits in an informal context and not for every kind of hit.

Nach der Beleidigung drohte ihm sein Bruder mit einer Schelle.
After the insult, his brother threatened him with a slap.
Beim Training übte Max einen kräftigen Schlag gegen den Sandsack.
During training, Max practiced a powerful blow against the punching bag.