っぽい (occasionally also seen as ぽい) is a 接尾語（せつびご） (suffix) that may be attached to many different types of words, in order to create new
い-Adjectives. In all cases,
っぽい indicates something that is ‘exhibiting characteristics of (A)’. This is most often in reference to the way (A) is acting, or the way (A) looks. In English,
っぽい may be translated as ‘(A)ish’, ‘(A)like’, or ‘tendency to (A)’.
っぽい quite often carries a negative connotation.
As
っぽい creates
い-Adjectives from other words, it may be used with the 連用形（れんようけい） (conjunctive form) of verbs, or the 語幹（ごかん） (stem form) of any other word.
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私（わたし）は飽（あ）きっぽいから、何（なに）も続（つづ）かない。
Because I have a tendency to get sick of things, I can't continue with anything.
白（しろ）っぽいやつをください。
Please give me the whiteish one.
タナカ君（くん）はファッション業界（ぎょうかい）では有名（ゆうめい）っぽいよ！
Apparently Tanaka-kun is popular in the fashion industry!
今（いま）の話（はなし）は嘘（うそ）っぽいけど本当（ほんとう）の話（はなし）なの。
The story I just told you sounds like a lie, but it is actually true.
As
っぽい creates new
い-Adjectives,
っぽい itself may be conjugated in exactly the same way as regular
い-Adjectives.
-
昔（むかし）は紫（むらさき）っぽかったのに今（いま）はなぜか赤（あか）っぽい。
Back in the day, it was purpleish, but now for some reason it is reddish.
この靴（くつ）なんか安（やす）っぽくない？
Don't these shoes look cheapish?
The fundamental meaning of っぽい will change depending on what type of word it is being used with. The general rules are as follows:
Verb + っぽい - To have a tendency to do (A).
Adjective + っぽい - To exhibit all the signs of being (A).
Noun + っぽい - To obviously not be (A), although show signs of being (A).
From this, we can see that っぽい is generally based on the opinion of the speaker, rather than the innate qualities of what is being observed. Thus, with verbs, adjectives, and nouns, the speaker feels like (A) is in excess, compared to what would be expected. This ‘being in excess of what is normal’ in the opinion of the speaker is what creates the negative connotation.