_ster?

[tingirl]

What is the significance of '_ster' in words such as lobster, monster, gangster?

[jno]

The Old English ending -estre, implying someone doing something or related to something. Lob comes from a word for spider so even that may qualify (but oyster doesn't, that's from the French).

[Quizmonster]

The 'estre' ending was originally just a feminine form. For example, a male weaver was a 'webba' whilst a female one was a 'webestre'...which is where our surname 'Webster' came from. By the 1300s, the 'ster' suffix was applied to both genders. The suffix just suggested that the person it was applied to was the one who performed some activity.
'Lobster' originally came from the Latin word 'locusta' meaning a locust. It was applied to the sea-creature because of a supposed similarity of shape.
'Monster', too, comes from a Latin word, 'monstrum'.
'Gangster' is the only one of your three words that fits the definition of the agent or doer of some activity.

[tingirl]

Thanks for your help :)

[jno]

the dictionary I had with me at work last night just gave lobster as derived from spider. Checking the Bloomsbury dictionary of word origins today: it gives locusta as QM says, but points out that it came into English twice, once as locust, once as lobster, and speculates that this may be 'due to the influence of' loppe, meaning spider - 'the Old English precursor of lobster was loppestre'. Presumably our ancestors saw a similarity between a lobster and a giant spider (ugh).

Modern formations like funster and jokester are probably created along the lines of words like spinster and gangster

[Octavius]

As QM says, the ending '-ster' is what grammarians call an agentive suffix, one that turns a verb for some activity into the name of a person who does it. Originally, it was always applied to a woman (though that has now changed), as in brewster (a woman who brews ale, a female job in a medieval household), maltster (a woman who makes the malt from which ale was brewed), and spinster (a woman who spins).

[tingirl]

Thanks everyone for your enlightening info. The spider and lobster connection was an eye-opener too!