
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
Loading your saved state.

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.
negligent
Both words are appropriate for careless behavior with potentially negative consequences. Fahrlässig emphasizes that the required care was not exercised. Leichtsinnig more strongly emphasizes knowingly accepting a recognizable danger without giving it sufficient thought.

Der Bootsführer handelte leichtsinnig, weil er trotz der Sturmwarnung aufs Wasser fuhr.
The boat operator acted recklessly because he went out on the water despite the storm warning.
Der Bootsführer handelte fahrlässig, weil er die Rettungswesten vor der Fahrt nicht kontrollierte.
The boat operator acted negligently because he failed to check the life jackets before setting out.