What does the term '死人不偿命' mean?
First, you should discern whether it's used as its literary meaning or metaphorical meaning from a given context. 

Literally, '死人不偿命' means one wouldn't compensate for other's death.  

Metaphorically, don't come to me (for reasoning) if you would 'die' of it. The typical example is 笑死人不偿命，for example 笑死人不偿命的笑话.  The similar phrase in English could be 'something that could laugh your ass-off'.  

On the other hand, 笑死人不偿命的笑话 sometimes could be a way of tempting, in which they suggest you not taking a shot of it otherwise it would 'kill' you by too much without any compensation. This could have the effect of the psychological inversion, and finally you wouldn't wait to take a shot of it. 

There are some other similar phrases in our daily usage, like 笑死人了，笑死了， 吵死了， 烦死了, ... . You could say 我最近烦死了！（I've been bored to death recently.）.