Haha, interesting question. It's literal translation is thick / big nerve. Nerves are usually thin and sensitive, so thick nerve is dull and insensitive. I believe the right word for it is INSENSITIVE in English. Sometimes people use this word for selfmockery.

You're right, it means "careless," ”slow on the uptake," "not sensitive to details." 神經大條就有一點像是反應比較遲鈍，做事沒有詳細思考或者是思考緩慢甚至有時候跟本沒在思考.

In oral english,神經大條 means insane, nuts, idiot. I'm 100% sure 'coz I just call myself that.

没心没肺，做事情马马虎虎，不仔细的意思。 也形容人的感觉不敏锐，不能很好地领会到他人的心情

This word has a lot means.It usually describes someone is careless,slow in reaction,lack of sophistication or subtlety.