BrEng: "pull your finger out", "cock up" and "stuff it" What do they mean?

In the British sitcom, The Thin Blue Line, Detective Grim makes three intelligently crafted sentences, which are given below. What do they mean?

It's my arse on the line, so you better pull your finger out

It's my arse on the line, and I don't want a cock up

It's my arse on the line, and if you stuff it, I will be red in the face

As Jo Bedard mentions in the comment to Sumit's answer, there are sexual overtones (they are too explicit to be called undertones indeed).

The general meaning of all three expressions is that the speaker's reputation and / or career may depend on the outcome of the current project or undertaking and he urges the other person not to contribute to a failure.

My arse is on the line is a not-so-polite variation on my career is on the line. Something being on the line means it is at risk. A different expression, very visual expression is my neck is on the block (which is used when there is an imminent risk of failure) or indeed my neck is on the line which roughly means “I am (at least figuratively) risking my neck for this.”

The explicit use of arse, meaning one's behind, gives the speaker (or, his script-writer — as detective Grim's character is seemingly unaware of the double entendres in his wordsmithery) the possibility to “enhance” on the imagery involved:

It's my arse on the line, so you better pull your finger out.

Pulling one's finger out (of their behind) means to get into action, to stop sitting around doing nothing useful. The combination with the speaker's arse being the one that is on the line implies that the addressee wasn't having his finger up his own posterior, but that of his boss. That can be seen as another anal reference to the habit some people have to try and please their bosses rather than doing useful work.

It's my arse on the line, and I don't want a cock up.

At face value, the sentence is simple: my job is at risk, and I don't want a disaster. A cock up is a mess up, a project turning into a disaster. However, the combination implies that we could read the sentence as It's my arse on the line, and I don't want a penis up that arse: my behind is at risk at them moment so don't fool around with it.

It's my arse on the line, and if you stuff it, I will be red in the face.

Again, stuff it normally means the same as causing a cock up: messing up the situation. The combination reads: my arse is on the line, if you put something up my arse, you will make me red in the face. Whether the redness implies a shamed blush or an angry flush because of the (unwanted) intimacy is up to the reader...

I hope it is needless to say that none of these expressions are recommended for use in a formal situation, and in most working environments will not at all be appreciated, whether said in jest or used as serious encouragement to your colleagues!