lustig
funny
Something that makes people laugh or is amusing; humorous.
Der Film war so lustig, dass wir die ganze Zeit gelacht haben.
The film was so funny that we laughed the whole time.

funny
Loading your saved state.
funny
Something that makes people laugh or is amusing; humorous.
Der Film war so lustig, dass wir die ganze Zeit gelacht haben.
The film was so funny that we laughed the whole time.

The adjective 'lustig' (funny/fun) is related to the noun 'die Lust' (desire/pleasure), which means joy or enjoyment. So something that is 'lustig' (funny/fun) gives pleasure or makes you laugh.
Die Kinder hatten große Lust auf ein Eis.
The children really felt like having an ice cream.
When 'lustig' (funny/strange) describes something unexpected or strange, it often has an ironic undertone. You can usually recognize this meaning from the situation or the emphasis.
Eine lustige Geschichte: Ich habe meinen Schlüssel gesucht und hatte ihn die ganze Zeit in der Hand.
Funny story: I was looking for my key and had it in my hand the whole time.
'Lustig' (fun/funny) often describes a good, enjoyable time, not just something that makes you laugh. A 'lustiger Abend' (fun evening) or a 'lustige Runde' (fun group/gathering) means that the atmosphere was cheerful and pleasant.
Danke für den lustigen Abend, es hat wirklich Spaß gemacht!
Thanks for the fun evening; it was really enjoyable!
serious
'ernst' and 'lustig' are often opposites when talking about mood or attitude. 'lustig' describes something that makes people laugh or has a cheerful mood. 'ernst' describes something without fun or joking; it can also mean that something is important or worrying. The words can be confused when a situation seems friendly but is still important. In that case, 'ernst' fits the meaning, and 'lustig' only fits the mood or the joke.

Nach der Arbeit erzählte Tom eine lustige Geschichte.
After work, Tom told a funny story.
Die Ärztin wurde ernst, als sie über die Untersuchung sprach.
The doctor became serious when she talked about the examination.