Mosquita muerta

[natty]

Today my spanish teacher told me that "Eres una mosquita muerta"... What does it mean? I asked her what does she mean, but she didn't reply.. Is it rude? :/

[wrholt]

It's not rude. It's a diminutive form of mosca muerta, which is an expression that means "hypocrite"; someone who says one thing but does something different.

[natty]

Am I hypocrite because I was speaking to my classmate? It doesn't make any sence to me... :/

Maybe I'm the one who is getting it wrong, because in Hungary if someone tells she is hypocrite, it means she is two-faced, insincere... In my opinion, that's a bit offensive... So I still don't understand why am I insincere for speaking to my classmate?

[AngelicaDeAlquezar]

It's not necessarily a hypocrite. It's someone whose personality seems to be quiet or paceful or benevolent and suddenly it turns out that they are unkind or lively or mean.

If you are usually silent or kind, and today you were talking loud or joking or giggling with your classmate, then the teacher had thought you were different from what you have just shown.

Although it may be said as a form of insult the way wrholt described it, it's not when one simply states that they had thought you have a different personality than the one they expected.

[chileno]

it might also mean "playing the fool" when you are not....

[wrholt]

Ah, good! On-line sources that I found gave mixed results. I also found mosca muerta in one of my bilingual dictionaries under the entry for mosca, but there it defined the expression as 'hypocrite', which is somewhat inaccurate given that it seems that 'hypocrite' has a stronger negative association than mosca muerta.

[natty]

Today I went to my teacher and asked why did she said so, and told me that when she was explaining I was chatting with my classmate but when she looked at me I acted like I didn't do anything, I was like a little angel who was paying attention.
I think now everything is clear ) Thank you for your help

[chileno]

That's what I meant by "playing (the) fool"

Haciéndose la tonta.

[JPablo]

Chileno... on point!

Additional...
From A Spanish-English Dictionary (Granada University, Spain), 10.2
mosquita muerta

= butter wouldn't melt in his mouth
Ex: Everytime I mention him though people say things like 'he is a naughty boy' but I don't know why I remember him as the butter wouldn't melt in his mouth type like the rest of us.

Oxford:
mosquita f: ser or parecer una mosquita muerta = to look as if butter wouldn’t melt in one’s mouth