But if you merely attempt to enforce the connotation of preciousness by pulling out a dictionary and looking up the definition of 'life', see the fallacy of the Argument from Common Usage; dictionary editors can't settle moral arguments.

M, they may be able to argue some, though it's a minority; for example if I promised to my grandmother on her deathbead to never eat fish on Tuesdays, than the morality of certain actions may hinge on the common usage definition of "fish".

Similarly, the morality of saying "I did not have sex with that woman" may depend on what is understood exactly by "sex" (not that a dictionary is necessarily the final arbiter!).

And more generally, rules and norms and laws may refer to words, and while the rules themselves should be evaluated on consequentialist grounds, judging whether one followed the rules may depend on common usage definitions.

For example, it's a nearly universal norm in western societies that racism is wrong. With the way humans are now, it's probably better than a situation where there was no norm against racism itself, but rather acts and beliefs were judged individually as right or wrong - that would leave too much leeway for rationalization. So instead we have the lesser evil of the definition of "racism" becoming overly broad and contested.

(Overall I mostly agree with you; definitions are totally useless on settling empirical disagreements, and mostly useless for moral disagreements)