Learn German: Du bist ein Feigling!

Yes! Any noun ending in the suffix -ling is predictably masculine. Because the verb sein (here as bist) connects the subject to a noun in the same case, we use the masculine nominative article ein.

How is the word Feigling constructed?

It combines the adjective feige (cowardly) with the suffix -ling. In German, adding -ling to an adjective or a verb stem is a common way to create a masculine noun describing a person characterized by that trait.

If I wanted to address a group and say 'You are cowards!', how does the plural of Feigling work?

You would say Ihr seid Feiglinge! Masculine nouns ending in -ling always form their plural simply by adding -e, without adding any extra umlauts.

I still sometimes mix up accusative and nominative. Why doesn't Feigling take the accusative einen here?

The verb sein (to be) is a linking verb, meaning it acts like an equal sign between the subject (du) and the noun (Feigling). Because the noun doesn't receive an action like a direct object would, it stays in the nominative case.