The term monogamish was coined by Dan Savage and defined as mostly monogamous but with a bit of wiggle room.
As with in all consensually non-monogamous relationships, you're able to deviate from the default of strict monogamy, and set your own rules and boundaries that suit yours and your partners needs. The term monogamish tends to be adopted for relationships that are mainly monogamous but allow for some exceptions.
So for example, a monogamish couple may choose to be non-monogamous only in the context of threesomes, or only when on holiday, or only with strangers, or only for oral sex. The options are limitless when you choose your own rules.