No, it is not safe. For a couple of reasons:
Generally speaking, charging lithium is already dangerous because it's not trivial to do it correctly but
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Since lithium's nominal voltage is about 3.7V, and you have something in the neighbourhood of 12, there are probably 3 in series (known as 3S). This means you need a charge ballancer, because without one, one cell will exceed its max voltage before the others and you'll have a fire.
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Since your "plug", which I take to be an AC adapter, is not a "smart" device, it will just dump as much current into the battery as it wants according to ohms law until the batt voltage approaches the charge voltage. Since most lithiums can't tolerate a charge rate higher than 1C, again, you will have a fire.
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Without any means of measuring, cell voltage, pack voltage, charge rate or temperature, you will probably over-charge the battery (above 4.2V) and again, you've got a fire.
In other words, don't do it. If you told me you had a Ni-Cd battery we'd be having a different discussion and I'd say go for it but with lithium, no.