leichtsinnig
reckless
Careless and without thinking about possible dangers or negative consequences.
Es war leichtsinnig von ihm, bei dem Gewitter schwimmen zu gehen.
It was reckless of him to go swimming during the thunderstorm.

reckless
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reckless
Careless and without thinking about possible dangers or negative consequences.
Es war leichtsinnig von ihm, bei dem Gewitter schwimmen zu gehen.
It was reckless of him to go swimming during the thunderstorm.

The word is made up of 'leicht' (light/easy) and 'Sinn' (sense/mind). Figuratively, it describes a 'light mind'—someone who does not take things seriously enough and is therefore careless.
Seine leichtsinnige Fahrweise hat schon zu mehreren Unfällen geführt.
His reckless driving style has already led to several accidents.
A very common construction is: 'Es ist/war leichtsinnig von [person in the dative], [etwas zu tun]' (It is/was reckless of [person] to [do something]). This evaluates a person's behavior.
Es war leichtsinnig von dir, ohne Jacke aus dem Haus zu gehen.
It was reckless of you to leave the house without a jacket.
'Leichtsinnig' (reckless) is often used to describe actions or characteristics. Typical combinations are 'leichtsinnig handeln' (to act recklessly), 'eine leichtsinnige Entscheidung' (a reckless decision), or 'leichtsinniges Verhalten' (reckless behavior).
Er hat in seiner Jugend viele leichtsinnige Entscheidungen getroffen.
He made many reckless decisions in his youth.
careful / cautious
'vorsichtig' and 'leichtsinnig' are clear opposites. 'vorsichtig' is used when someone acts attentively and thinks about possible dangers. 'leichtsinnig' is used when someone is careless and thinks too little about the consequences. 'leichtsinnig' often sounds like criticism because the person is taking an unnecessary risk. In short: caution protects against problems; recklessness can cause problems.

Sie ist leichtsinnig, wenn sie ohne Helm mit dem Fahrrad durch die Stadt fährt.
She is reckless when she rides her bike through the city without a helmet.
Am Bahnhof seid ihr vorsichtig, weil dort viele Menschen unterwegs sind.
At the train station, you are careful because there are many people around.