
unsinnig
nonsensical
Without sense, purpose, or logic; absurd or foolish.
Es ist unsinnig, bei diesem Sturm an den Strand zu gehen.
It's nonsensical to go to the beach in this storm.
nonsensical
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nonsensical
Without sense, purpose, or logic; absurd or foolish.
Es ist unsinnig, bei diesem Sturm an den Strand zu gehen.
It's nonsensical to go to the beach in this storm.
The word consists of the prefix 'un-', which expresses negation, and the adjective 'sinnig' (sensible). So 'unsinnig' literally means 'without sense'.
Deine Argumentation ist völlig unsinnig.
Your reasoning is completely nonsensical.
'Unsinnig' is very often used in the structure 'Es ist unsinnig, etwas zu tun' (It is nonsensical to do something). This construction with 'zu' + infinitive is a typical way to judge an action as pointless.
Es ist unsinnig, ohne Ticket zum Konzert zu fahren.
It's nonsensical to go to the concert without a ticket.
'Unsinnig' often describes abstract things. Typical combinations are 'eine unsinnige Regel' (a nonsensical rule), 'eine unsinnige Frage' (a nonsensical question), or 'eine unsinnige Diskussion' (a nonsensical discussion).
Das ist eine unsinnige Regel, die niemand versteht.
That is a nonsensical rule that nobody understands.
pointless / senseless
Sinnlos emphasizes that an action has no reasonable purpose or benefit. Unsinnig focuses more on a lack of logic, contradiction, or absurdity. Both can fit an unreasonable action; the key question is whether you are mainly criticizing its purpose or its logic.

Es ist unsinnig, im Dunkeln nach dem Schlüssel zu suchen und dabei die Lampe ausgeschaltet zu lassen.
It is nonsensical to look for the key in the dark while leaving the lamp switched off.
Es ist sinnlos, weiter nach dem Schlüssel zu suchen, denn er liegt schon auf dem Tisch.
There is no point in continuing to look for the key because it is already on the table.