1. To fill some container or vessel with more of something. A noun or pronoun can be used between "top" and "up." Often used without directly stating the thing being filled, with the person or thing it belongs to being mentioned instead. Oh, you're glass is nearly empty, Tom. Let me top you up. Would you bring this bottle of wine to the table and top up everyone's glass, please? Make sure you top the car up with gas before you set out.

2. To add more money to some account. With our pay-as-you-go cell phone plans, you can top up over the phone or at various stores around the country. Don't forget to top up your account regularly, as payments are taken from it automatically.

3. To supplement one's income. A noun or pronoun can be used between "top" and "up." I do a bit of freelance work in the evenings to top up what I earn each month. You'll be on a lower starting wage, but you can top it up with commissions.