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.
sensible
'sinnvoll' and 'unsinnig' are opposites. Use 'sinnvoll' when something has a clear purpose or seems sensible. Use 'unsinnig' when something makes no sense, is not logical, or even seems foolish. 'unsinnig' often sounds stronger than 'sinnlos', because it can express not only a lack of usefulness, but also a stupid or absurd idea.

In der Nachbarschaft haben viele den unsinnigen Vorschlag sofort abgelehnt.
Many people in the neighborhood immediately rejected the nonsensical suggestion.
Vor dem Termin ist es sinnvoll, alle Unterlagen noch einmal zu prüfen.
Before the appointment, it is sensible to check all the documents once more.