Words mean business. Every one of the them represents, by default, 100% of the meaning it's associated with: the word "apple" brings to mind a whole, typical, orthodox apple—not an "almost apple", "about apple", or "awkward apple". To add these further shades of meaning we normally have to combine the basic word with adjectives or other descriptive elements.
Japanese has diverse ways of fulfilling such functions. One of those handy vocabulary gadgets is the word 半分, which as a standalone noun means "half", but can also be attached as a suffix to other words in order to weaken their meaning and suggest something other than the full-on, typical thing we think about when we see that word.
This suffix can only be attached to nouns, including the noun form of verbs (ex. 遊ぶ → 遊び). The new combination of noun+suffix is itself a noun, which can be turned into an adverb with the particle に or into an adjective with の.
The nuance added by the suffix 半分 is one of being halfhearted, not entirely serious or committed. In this function it corresponds to English prefixes such "half-", "semi-", and "part-". Common combinations include 冗談半分に, "half-jokingly" and 面白半分に, "half-seriously", "just for the fun of it". Let's see how another such combination is used in a sentence from Reajer book 5, The Samurai and the Ghost Light:
The samurai started to follow it out of idle curiosity.
興味 is "interest", and its combination with 半分, when used with に, can be translated as above, or as any similar variation such as "just to see what would happen" or even "casually". The idea here is that the samurai acted without making much of the situation, and wasn't trying very hard to accomplish a predefined goal.
The selection of nouns that can interact like this with 半分 is fairly limited, because the combination must convey some idea of intention or feeling. But the combinations that do exist are quite common in Japanese, and you'll frequently see them in the spoken as well as the written language.
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